Method for forming conductive pattern and wiring board

ABSTRACT

[Problem] To provide a conductive pattern formation method in which a fine pattern can be formed in a simple way at low cost. 
     [Means for Solving Problem] A flat plate having a convex pattern on its surface is provided so as to oppose a substrate, a fluid body including conductive particles and a gas bubble generating agent is supplied into a gap between the substrate and the flat plate, and thereafter, the fluid body is heated for generating gas bubbles from the gas bubble generating agent included in the fluid body. The fluid body is forced out of the gas bubbles as the gas bubbles generated from the gas bubble generating agent grow, so as to self-assemble between the convex pattern formed on the flat plate and the substrate owing to interfacial force, and an aggregate of the conductive particles included in the fluid body having self-assembled is made into a conductive pattern formed on the substrate.

RELATED APPLICATIONS

This application is the U.S. National Phase under 35 U.S.C. § 371 ofInternational Application No. PCT/JP2006/315956, filed on Aug. 11, 2006,which in turn claims the benefit of Japanese Application No.2005-254773, filed on Sep. 2, 2005 the disclosures of which Applicationsare incorporated by reference herein.

TECHNICAL FIELD

The present invention relates to a method for forming a conductivepattern on a substrate, and more particularly, it relates to a methodfor forming a conductive pattern in which a fine pattern can be obtainedin a simple way.

BACKGROUND ART

In the case where a conductive pattern (a wiring pattern) is formed on asubstrate such as a printed board, photolithography and etching areperformed in general, and since these processes include a large numberof complicated procedures, time and labor are necessary for forming aconductive pattern in general.

As a technique to form a fine conductive pattern without performing suchprocesses, a method in which a conductive pattern is drawn on asubstrate by using a conductive paste including metal nano-particles andthe thus obtained conductive paste layer is heated for forming a desiredconductive pattern is known (for example, Patent Documents 1, 2 and thelike).

As a general method for drawing a conductive pattern, a screen maskprovided with an opening is used as a template for applying a conductivepaste in a desired thickness by a screen printing method (for example,Patent Document 1 and the like). Apart from this, a method in which aconductive paste is directly sprayed by an inkjet method for drawing adesired conductive pattern has been developed (for example, PatentDocument 2 and the like).

Patent Document 1: Japanese Laid-Open Patent Publication No. 2004-247572

Patent Document 2: Japanese Laid-Open Patent Publication No. 2002-134878

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

The method for forming a conductive pattern described in each of PatentDocuments 1 and 2 is simpler than the conventional complicated processesincluding the photolithography and the etching, and hence, a conductivepattern can be formed at a comparatively low cost.

However, it is difficult to form a fine pattern by the screen printingmethod, and there remain problems to be solved such as adhesion betweena conductive paste and a substrate in the inkjet method. Thus, any ofthese methods has not become a principal technique to be replaced withthe conventional conductive pattern formation method including thephotolithography and the etching.

The present invention was devised in consideration of thesecircumstances, and an object of the invention is providing a conductivepattern formation method in which a fine pattern can be formed in asimple way at a low cost.

Means for Solving Problems

The conductive pattern formation method of this invention is a methodfor forming a conductive pattern on a substrate and includes a firststep of providing a flat plate having a convex pattern on a surfacethereof in facing relation to the substrate; a second step of supplyinga fluid body containing conductive particles and a gas bubble generatingagent into a gap between the substrate and the flat plate; and a thirdstep of generating gas bubbles from the gas bubble generating agentcontained in the fluid body by heating the fluid body, and in the thirdstep, the fluid body is forced out of the gas bubbles generated from thegas bubble generating agent as the gas bubbles grow and self-assemblesbetween the convex pattern formed on the flat plate and the substrateowing to interfacial force, and the conductive pattern formed on thesubstrate is made of an aggregate of the conductive particles containedin the fluid body having self-assembled.

In a preferred aspect, the fluid body is made of a resin, and the thirdstep includes a sub-step of curing the resin after allowing the resin toself-assemble between the convex pattern and the substrate.

In a preferred aspect, the resin is a light setting resin and the resinhaving self-assembled between the convex pattern and the substrate iscured with light by selectively irradiating the resin with light in thethird step.

In a preferred aspect, the flat plate is a transparent substrate, and alight masking film is preferably formed in a surface area of the flatplate other than the convex pattern.

In a preferred aspect, the conductive pattern is made of the aggregateof the conductive particles in which the conductive particles are incontact with one another.

In a preferred aspect, the third step includes a sub-step of heating thefluid body at a temperature at which the conductive particles are meltedafter allowing the fluid body to self-assemble between the convexpattern and the substrate, and the conductive particles are bonded toone another through metallic bond in the sub-step of heating.

A melting point of the conductive particles is preferably higher than aboiling point of the gas bubble generating agent.

In a preferred aspect, the third step includes a sub-step of pressingthe flat plate against the substrate after allowing the fluid body toself-assemble between the convex pattern and the substrate, and theconductive particles are contact bonded to one another in the sub-stepof pressing.

In a preferred aspect, the gas bubble generating agent is made of amaterial that boils when the fluid body is heated or a material thatproduces a gas through thermal decomposition.

The gas bubble generating agent is preferably made of two or more kindsof materials having different boiling points.

In a preferred aspect, the third step is performed while varying the gapbetween the substrate and the flat plate.

In a preferred aspect, at least a surface of the convex pattern is madeof a metal.

In a preferred aspect, the third step includes a sub-step of filling asealant in the gap between the substrate and the flat plate afterallowing the fluid body to self-assemble between the convex pattern andthe substrate and curing the sealant after filling.

In a preferred aspect, the convex pattern is formed as a convex patternhaving at least two kinds of portions with different heights.

A portion with a smaller width is preferably higher than a portion witha larger width in the convex pattern.

In a preferred aspect, the substrate is a wiring board, and theconductive pattern corresponds to at least a part of a wiring patternformed on the wiring board.

In a preferred aspect, the method further includes a step of removingthe flat plate after the third step.

The conductive pattern preferably has a cross-section in a shape of anhourglass.

The wiring board of this invention is a wiring board on which a wiringpattern is formed, and the wiring pattern is formed by providing a flatplate having a convex pattern on a surface thereof in facing relation tothe wiring board, supplying a fluid body including conductive particlesand a gas bubble generating agent into a gap between the wiring boardand the flat plate, and heating the fluid body for allowing the fluidbody to self-assemble between the convex pattern formed on the flatplate and the wiring board, whereby forming the wiring pattern made ofan aggregate of the conductive particles included in the fluid bodyhaving self-assembled.

In a preferred aspect, in the wiring pattern made of the aggregate ofthe conductive particles, the conductive particles are bonded to oneanother through metallic bond by heating the fluid body havingself-assembled between the convex pattern and the wiring board.

EFFECT OF THE INVENTION

In the conductive pattern formation method of this invention, gasbubbles are generated from a gas bubble generating agent included in afluid body by heating the fluid body supplied into a gap between asubstrate and a flat plate, and the fluid body is forced out of the gasbubbles by the growing gas bubbles, so that the fluid body can beallowed to self-assemble between a convex pattern formed on the flatplate and the substrate owing to the interfacial force. As a result, anaggregate of the conductive particles included in the fluid body havingself-assembled forms a conductive pattern, and thus, the conductivepattern can be easily formed in a simple way of heating.

Furthermore, since the conductive pattern is formed in a self-assemblymanner in accordance with the convex pattern, the conductive pattern canbe formed in a fine shape.

In addition, when a curable material such as a resin is used as thefluid body, the conductive pattern made of the aggregate of theconductive particles can attain a structure stable in the strength bycuring the fluid body of the resin or the like after forming theconductive pattern through the self-assembly.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A through 1D are cross-sectional views for showing basicprocedures in a bump formation method utilizing self-assembly of aresin.

FIGS. 2A through 2D are cross-sectional views for showing other basicprocedures in the bump formation method utilizing the self-assembly of aresin.

FIGS. 3A and 3B are diagrams for explaining the mechanism of theself-assembly of a resin.

FIGS. 4A through 4E are cross-sectional views for showing basicprocedures in a conductive pattern formation method of this invention.

FIG. 5 is a perspective view for showing the structure of a conductivepattern formed on a substrate in the invention.

FIGS. 6A through 6E are cross-sectional views for showing procedures ina conductive pattern formation method according to another embodiment ofthe invention.

FIGS. 7A through 7C are cross-sectional views for showing procedures ina conductive pattern formation method according to still anotherembodiment of the invention.

FIGS. 8A through 8C are cross-sectional views for showing procedures ina conductive pattern formation method according to still anotherembodiment of the invention.

FIG. 9 is a diagram for showing materials of conductive particles ofthis invention.

FIG. 10 is a diagram for showing materials of a gas bubble generatingagent of this invention.

FIG. 11 is a diagram for showing materials of a gas bubble generatingagent powder of this invention.

Description of Reference Numerals 11,31 substrate 12,40 flat plate 13convex pattern 14 fluid body 16 conductive particles (solder powder) 18conductive pattern 19 bump 20,30 gas bubble 21 sealant 22 light 32electrode

BEST MODE FOR CARRYING OUT THE INVENTION

The present inventor has made various examinations on a method forforming a bump through self-assembly of conductive particles (such as asolder powder) on an electrode of a wiring board or a semiconductorchip, or on a flip-chip mounting method by forming a connecting bodybetween electrodes of a wiring body and a semiconductor chip throughself-assembly of conductive particles between the electrodes, and hasproposed a novel bump formation method and a novel flip-chip mountingmethod (Japanese Patent Application No. 2005-094232).

FIGS. 1A through 1D and 2A through 2D are diagrams for showing basicprocedures in the bump formation method disclosed by the presentinventor in the aforementioned patent application. The bump formationmethod alone will be herein described because the basic procedures arecommon to the flip-chip mounting method.

First, as shown in FIG. 1A, a fluid body (resin) 14 including conductiveparticles (solder powder) 16 and a gas bubble generating agent (notshown) is supplied onto a substrate 31 having a plurality of electrodes32 thereon. Next, as shown in FIG. 1B, a flat plate 40 is provided onthe surface of the fluid body (resin) 14.

When the fluid body (resin) 14 is heated under this condition, gasbubbles 30 are generated from the gas bubble generating agent includedin the fluid body (resin) 14 as shown in FIG. 1C. Then, the fluid body(resin) 14 are forced out of the gas bubbles 30 as the gas bubbles 30thus generated grow as shown in FIG. 1D.

The fluid body (resin) 14 thus forced out self-assembles in the shape ofa column on an interface with the electrode 32 of the substrate 31 andon an interface with the flat plate 40 as shown in FIG. 2A. Next, whenthe fluid body (resin) 14 is further heated, the conductive particles(solder powder) 16 included in the fluid body (resin) 14 are melted, andhence, the conductive particles (solder powder) 16 included in the fluidbody (resin) 14 having self-assembled on the electrode 32 are bonded toone another through melting.

Since the electrode 32 has high wettability against the conductiveparticles (solder powder) 16 bonded to one another through melting, abump 19 made of the melted solder powder is formed on the electrode 32as shown in FIG. 2C. Ultimately, when the fluid body (resin) 14 and theflat plate 40 are removed, the substrate 31 having the bump 19 formed onthe electrode 32 can be obtained as shown in FIG. 2D.

As a characteristic of this method, the fluid body (resin) 14 suppliedbetween the substrate 31 and the flat plate 40 is heated so as togenerate the gas bubbles 30, the fluid body (resin) 14 is forced out ofthe gas bubbles 30 as the gas bubbles 30 grow, and thus the fluid body(resin) 14 is allowed to self-assemble between the electrode 32 of thesubstrate 31 and the flat plate 40.

It is noted that the procedure for melting the solder powder by heatingthe fluid body (resin) 14 again after allowing the fluid body (resin) 14to self-assemble on the electrode 32 is performed in order to ultimatelyform the bump 19 on the electrode 32.

The phenomenon that the fluid body (resin) 14 self-assembles on theelectrode 32 seems to be caused through a mechanism shown in FIGS. 3Aand 3B.

FIG. 3A is a diagram for showing the state where the fluid body (resin)14 is forced out onto the electrode 32 of the substrate 31 by a growinggas bubble (not shown). The fluid body (resin) 14 thus brought intocontact with the electrode 32 is applied with stress Fs(∝γ) by theinterfacial force γ caused on the interface (the force derived from whatis called wet spread of the resin) and is spread over the electrode 32by the capillarity, and ultimately, the column-shaped fluid body (resin)having the boundary at the end of the electrode 32 is formed between theelectrode 32 and the flat plate 40.

Although the column-shaped fluid body (resin) 14 formed on the electrode32 through the self-assembly (hereinafter referred to as the “resincolumn”) is applied with stress F_(b) derived from the growth (or themovement) of the gas bubble 30 as shown in FIG. 3B, it can keep itsshape owing to the function of stress F_(η)(∝η) derived from theviscosity η of the fluid body (resin) 14.

At this point, whether the fluid body (resin) 14 having self-assembledcan keep a given shape depends upon not only the interfacial force γ butalso an area S of the electrode 32, a length L of the gap between theelectrode 32 and the flat plate 40 and the viscosity η of the fluid body(resin) 14.

When the resin column is kept in a substantially cylindrical shape, theLaplace pressure Δp is represented as follows with the radius of thecylindrical shape of the fluid body (resin) indicated by R and thewetting angle of the resin indicated by θ:

$\begin{matrix}{{\Delta\; p} = {\gamma\left( {\frac{1}{R} - \frac{\cos\;\theta}{L/2}} \right)}} & \left\lbrack {{Formula}\mspace{20mu} 1} \right\rbrack\end{matrix}$

On the basis of this formula, it is understood that the resin column isin the shape of an hourglass and functions as attractive force whenθ<2/L. Also, the stress Fs is represented as follows:F _(S) =πR ² Δp+2πRγ cos θ  [Formula 2]

For example, Fs=−13 [μN] when γ=40 [mN/m], R=50 [μm], L=20 [μm] andθ=0[°]. Alternatively, Fs=−25 [μN] when γ=80 [mN/m], R=50 [μm], L=20[μm] and θ=0[°]. Thus, the stability of the resin column can beincreased by increasing the interfacial force γ.

Furthermore, a resin is controlled in accordance with the followingReynolds equation:

$\begin{matrix}{{\frac{\partial}{\partial x}\left( {h^{3}\frac{\partial p}{\partial x}} \right)} = {12\eta\frac{\partial h}{dt}}} & \left\lbrack {{Formula}\mspace{14mu} 3} \right\rbrack\end{matrix}$

Therefore, the stability of the resin column can be increased also byincreasing the viscosity η.

As described above, the fluid body (resin) 14 is formed on the electrode32 in a self-aligned manner by utilizing the self-assembly of the fluidbody (resin) 14 caused by the interfacial force in this method. It canbe said that the self-assembly caused by the interfacial force utilizesa phenomenon occurring merely on the electrode 32 on which the gapbetween the substrate 31 and the flat plate 40 is smaller because theelectrode 32 formed on the substrate 31 is in a convex shape.

In the bump formation method, since the electrode is originally formedon the substrate in a convex shape, the fluid body (resin) naturallyself-assembles on the electrode. The present inventor has noticed thatif an arbitrary convex pattern is previously formed on a flat plate, afluid body (resin) can be formed in a self-assembly manner in accordancewith the convex pattern.

Specifically, when a convex pattern is formed on a flat plate in thesame pattern as a desired conductive pattern, the desired conductivepattern can be formed on a substrate by utilizing the self-assembly of afluid body (resin) caused by the interfacial force.

Now, preferred embodiments of the invention will be described withreference to the accompanying drawings. In the drawings mentioned below,the same reference numerals are used to refer to elements havingsubstantially the same functions for simplifying the description. It isnoted that the present invention is not limited to the embodimentsdescribed below.

FIGS. 4A through 4E are diagrams for showing basic procedures in aconductive pattern formation method according to an embodiment of theinvention.

First, as shown in FIG. 4A, a fluid body 14 including conductiveparticles 16 and a gas bubble generating agent (not shown) is suppliedonto a substrate 11. Next, as shown in FIG. 4B, a flat plate 12 having aplurality of convex patterns 13 is provided on the surface of the fluidbody 14 so as to oppose the substrate 11.

At this point, the convex patterns 13 are formed on the flat plate 12 inthe same layout as a conductive pattern to be formed on the substrate11. Also, in the case where the substrate 11 is, for example, a wiringboard on which electronic components and the like are built, the flatplate 12 is provided above the substrate 11 with necessary alignment.

It is noted that these procedures may be performed by previouslyproviding the substrate 11 and the flat plate 12 so as to oppose eachother with a given gap and then supplying the fluid body 14 includingthe conductive particles 16 and the gas bubble generating agent into thegap therebetween.

When the fluid body 14 is heated under this condition, gas bubbles 20are generated from the gas bubble generating agent included in the fluidbody 14 as shown in FIG. 4C. At this point, the fluid body 14 is forcedout of the gas bubbles 20 as the gas bubbles 20 grow.

The fluid body 14 thus forced out self-assembles between the convexpatterns 13 formed on the flat plate 12 and the substrate 11 owing tothe interfacial force as shown in FIG. 4D. When the flat plate 12 isremoved, patterns of the fluid body 14 obtained through theself-assembly is formed on the substrate 11.

At this point, an aggregate of the conductive particles 16 included inthe fluid body 14 having self-assembled forms the conductive patternswith the conductive particles 16 in contact with one another.

FIG. 5 is a perspective view of the conductive patterns 18 formed on thesubstrate 11 through the procedures shown in FIGS. 4A through 4E.Although the conductive patterns 18 are in a linear shape with aconstant width in FIG. 5, the width may be varied in the middle.Alternatively, not only a linear pattern but also a bent pattern or acrossing pattern can be formed. In this case, in order to easily keepthe shape of the fluid body 14 having self-assembled, each corner of theconductive patterns may be designed to have inclination of 45 degrees ora curvature.

In the case where the substrate 11 is a wiring board, the conductivepatterns 18 correspond to wiring patterns.

According to this invention, gas bubbles are generated from the gasbubble generating agent included in the fluid body 14 by heating thefluid body 14 supplied into the gap between the substrate 11 and theflat plate 12, and the fluid body 14 is forced out of the gas bubbles bythe growing gas bubbles, so that the fluid body 14 can be allowed toself-assemble between the convex patterns 13 formed on the flat plate 12and the substrate 11 owing to the interfacial force. As a result, theaggregate of the conductive particles 16 included in the fluid body 14having self-assembled forms the conductive patterns 18, and thus, theconductive patterns 18 can be easily formed in a simple way of heating.

Furthermore, since the conductive patterns 18 are formed in accordancewith the convex patterns 13 in a self-assembly manner, the conductivepatterns 18 can be formed in fine shapes.

Moreover, since each conductive pattern formed in this invention has across-section typically in the shape of an hourglass (having a neck inthe middle), the adhesiveness to the substrate can be improved ascompared with a wiring pattern (typically having a rectangular ortrapezoidal cross-section) formed by the conventional method includingthe etching and the like. Also, since its face opposite to the face onthe side of the substrate is formed in substantially the same size asthe face on the side of the substrate, also when the conductive patternis to be connected to, for example, a gold bump formed on asemiconductor chip, the connection area can be large, so as to realizehighly reliable semiconductor mounting.

It is noted that the sizes of the respective elements and the relativepositional relationship among them (for example, the size of theconductive particles 16 and the length of the gap between the substrate11 and the flat plate 12) shown in FIGS. 4A through 4E and 5 are givenmerely for the sake of convenience and do not correspond to the actualsizes and the like.

Next, a conductive pattern formation method in using a light settingresin as the fluid body 14 will be described with reference to FIGS. 6Athrough 6E.

As shown in FIG. 6A, a flat plate 12 having convex patterns 13 thereonis provided so as to oppose a substrate 11, and a fluid body (lightsetting resin) 14 including conductive particles 16 and a gas bubblegenerating agent (not shown) is supplied into a gap between thesubstrate 11 and the flat plate 12.

When the fluid body (light setting resin) 14 is heated under thiscondition, gas bubbles 20 are generated from the gas bubble generatingagent included in the fluid body (resin) 14 as shown in FIG. 6B. At thispoint, the fluid body (resin) 14 is forced out of the gas bubbles 20 asthe generated gas bubbles 20 grow.

The fluid body (resin) 14 thus forced out self-assembles between theconvex patterns 13 formed on the flat plate 12 and the substrate 11owing to the interfacial force as shown in FIG. 6C.

Next, as shown in FIG. 6D, the fluid body (light setting resin) 14 isirradiated with light 22 of UV or the like through the substrate 11. Atthis point, when the substrate 11 is made of a transparent member (suchas glass) and has a light masking film 23 (of, for example, a chromiumfilm) formed on its surface area other than the convex patterns 13, thefluid body (light setting resin) 14 having self-assembled can beselectively irradiated with the light, and as a result, the fluid body(light setting resin) 14 thus irradiated with the light can beselectively cured.

Thereafter, as shown in FIG. 6E, the flat plate 12 is removed, so as toform patterns of the fluid body (light setting resin) 14 obtainedthrough the self-assembly, namely, conductive patterns 18 made of anaggregate of the conductive particles 16, on the substrate 11.

In this manner, when the fluid body (light setting resin) 14 is curedafter forming the conductive patterns 18 through the self-assembly, theconductive patterns 18 made of the aggregate of the conductive particles16 can be formed in a structure stable in the strength.

Although the light setting resin is herein used, a thermosetting resinmay be used instead. In the case where a thermosetting resin is used,the contact among the conductive particles 16 can be made strongerthrough shrinkage of the resin caused during the thermal curing.

Furthermore, when the fluid body (resin) is heated to a temperature atwhich the conductive particles 16 are melted after the self-assembly ofthe fluid body (resin) 14, the conductive particles 16 are bonded to oneanother through metallic bond, and thus, the conductive patterns can beformed in a structure stable in the strength as well as the resistancevalue of the conducive patterns can be made smaller.

It is noted that when a thermosetting resin is used as the fluid body14, the conductive particles 16 are melted at the same time as the resinis cured through the aforementioned heat treatment.

FIGS. 7A through 7C are cross-sectional views for showing exemplifiedprocess for allowing the fluid body 14 to self-assemble between theconvex patterns 13 and the substrate 11, filling a sealant (of, forexample, a resin) in the gap between the substrate and the flat plate 12and curing the sealant.

FIG. 7A is a diagram for showing a state attained after the procedure ofFIG. 4D in the process shown in FIGS. 4A through 4E, namely, a statewhere the fluid body 14 has self-assembled between the convex patterns13 and the substrate 11.

Under this condition, a sealant 21 (of, for example, a resin) is filledin the gap between the substrate 11 and the flat plate 12 as shown inFIG. 7B, and the sealant 21 is cured. Thereafter, the flat plate 12 isremoved, and projecting portions of the sealant 21 are cut off so as toset the surface of the sealant 21 at the same level as that of theconductive patterns 18 as shown in FIG. 7C.

As shown in FIG. 7C, the conductive patterns 18 (the fluid body 14)formed on the substrate 11 through the self-assembly are sealed with thesealant 21, and thus, the structure of the conductive patterns 18 can bemade rigid and the conductive patterns 18 can be protected by thesealant 21.

According to the conductive pattern formation method of this invention,a conductive pattern can be formed in an arbitrary shape, andfurthermore, conductive patterns having different heights (thicknesses)can be simultaneously formed. This will be described with reference toFIGS. 8A and 8B.

FIG. 8A is a diagram for showing a procedure for supplying a fluid body14 including conductive particles 16 and a gas bubble generating agent(not shown) into a gap between a substrate 11 and a flat plate 12 andheating the fluid body 14, and this procedure is different from theprocedure shown in FIG. 4C in convex patterns 13 a and 13 b withdifferent heights formed on the flat plate 12.

As shown in FIG. 8A, gas bubbles 20 are generated from the gas bubblegenerating agent included in the fluid body 14, the fluid body 14 isforced out of the gas bubbles 20 as the gas bubbles 20 grow and thefluid body 14 thus forced out self-assembles between the convex patterns13 a and 13 b and the substrate 11 owing to the interfacial force asshown in FIG. 8B.

As a result, when the flat plate 12 is removed, conductive patterns 18 aand 18 b (fluid bodies 14 a and 14 b) with different heights are formedon the substrate 11 as shown in FIG. 8C.

This is for the following reason: Portions of the fluid body 14respectively in contact with the surfaces of the convex patterns 13 aand 13 b are wholly spread over the convex patterns 13 a and 13 b owingto the interfacial force obtained on the interfaces, and thereafter, theportions of the fluid body 14 can keep their shapes due to the functionof the stress caused by the viscosity of the fluid body 14.

On the basis of Formula 1 described above, in order to improve thestability of the column-shaped fluid body (resin column) 14 formedthrough the self-assembly, among the convex patterns 13 a and 13 bhaving the different heights, a pattern with a smaller width (namely,the convex pattern 13 a) preferably has a larger height than a patternwith a larger width (namely, the convex pattern 13 b).

Since the conductive patterns with the different heights and widths canbe thus simultaneously formed, in the case where the conductive patternsare used as, for example, signal lines, impedance can be easily matched.

Moreover, when a release agent such as an acrylic resin is previouslyapplied on a substrate and a conductive pattern is formed on thesubstrate by the method of this invention so as to transfer theconductive pattern onto another member, the present method is applicableto formation of a microwave circuit conventionally difficult to form. Atthis point, an adhesive material is preferably applied previously to themember onto which the pattern is transferred.

Furthermore, when conductive patterns partially having different heightsare formed on a substrate by the method of this invention and anothersubstrate having a wiring pattern thereon is stacked on the substrate tobe pressed at a given pressure, wirings are connected to each otherbetween these substrates through a wiring partially having a largerheight. At this point, when the surface of the substrate on which theconductive patterns are formed is placed in a semi-cured state, amultilayered wiring board can be obtained by curing this substrate afterconnecting the wirings between the substrates.

Herein, the fluid body 14, the conductive particles 16 and the gasbubble generating agent used in the conductive pattern formation methodof this invention are not particularly specified, and followingmaterials can be respectively used:

The fluid body 14 may be any fluid body having given viscosity to theextent that it is fluid in a temperature range from room temperature tothe melting temperature of the conductive particles 16, and may be afluid body having viscosity lowered to be fluid by heating. Typicalexamples are a thermosetting resin such as an epoxy resin, a phenolresin, a silicone resin, a diallyl phthalate resin, a furan resin or amelamine resin, a thermoplastic resin such as polyester estramer, afluororesin, a polyimide resin, a polyamide resin or an aramid resin, alight (UV) setting resin or the like, and a combination of any of them.Apart from these resins, a high-boiling solvent, an oil or the like canbe used.

Furthermore, materials shown in FIGS. 9 and 10 can be appropriatelycombined to be used as the conductive particles 16 and the gas bubblegenerating agent. When the melting point of a material of the conductiveparticles 16 is higher than the boiling point of a material of the gasbubble generating agent, it is possible to perform, after allowing thefluid body to self-assemble by generating gas bubbles from the gasbubble generating agent by heating the fluid body 14, a procedure forfurther heating the fluid body 14 for melting the conductive particlesincluded in the fluid body having self-assembled so as to bond theconductive particles to one another through the metallic bond.

Moreover, the gas bubble generating agent may be made of two or morekinds of materials having different boiling points. When the boilingpoints are different, there arises a difference in timing of thegeneration and the growth of the gas bubbles, and as a result, the fluidbody 14 is forced out through the growth of the gas bubbles in astepwise manner. Therefore, the process of the self-assembly of thefluid body 14 can be made homogeneous, so that conductive patterns withmore homogeneity can be formed.

Apart from the materials shown in FIG. 10, a material for generating gasbubbles through thermal decomposition of the gas bubble generating agentin heating the fluid body 14 can be used as the gas bubble generatingagent. As such a gas bubble generating agent, any of materials shown inFIG. 11 can be used. For example, in the case where a compound includingwater of crystallization (aluminum hydroxide) is used, it is thermallydecomposed when the fluid body 14 is heated, so as to generate steam inthe form of gas bubbles.

Although the present invention has been described in preferredembodiments, it goes without saying that the description is notrestrictive but the disclosed invention may be modified in numerousways. For example, after allowing the fluid body to self-assemblebetween the convex patterns and the substrate, the flat plate may bepressed against the substrate for contact bonding the conductiveparticles.

Alternatively, the procedure for allowing the fluid body toself-assemble between the convex patterns and the substrate may beperformed while changing the gap between the substrate and the flatplate. Thus, the fluid body can be made to efficiently self-assemblebetween the convex patterns and the substrate.

Moreover, at least surface portions of the convex patterns may be madeof a metal. Thus, the fluid body can be made more easily self-assemblebecause the interfacial force is varied on the metal surface.Furthermore, in the case where the conductive patterns are formedthrough the metallic bond of the conductive particles by melting theconductive particles, the melted conductive particles can be easilybonded to the metal face with high wettability, so as to further reducethe resistance value.

INDUSTRIAL APPLICABILITY

According to the present invention, a conductive pattern formationmethod in which a fine pattern can be formed in a simple way at low costis provided.

1. A method for forming a conductive pattern on a substrate comprising:a first step of providing a flat plate having a convex pattern on asurface thereof in facing relation to the substrate; a second step ofsupplying a fluid body containing conductive particles and a gas bubblegenerating agent into a gap between the substrate and the flat plate;and a third step of generating gas bubbles from the gas bubblegenerating agent contained in the fluid body by heating the fluid body,in the third step, the fluid body being forced out of the gas bubblesgenerated from the gas bubble generating agent as the gas bubbles growand self-assembling between the convex pattern formed on the flat plateand the substrate owing to interfacial force, and the conductive patternformed on the substrate being made of an aggregate of the conductiveparticles contained in the fluid body having self-assembled.
 2. Themethod for forming a conductive pattern on a substrate of claim 1,wherein the fluid body is made of a resin, and the third step includes asub-step of curing the resin after allowing the resin to self-assemblebetween the convex pattern and the substrate.
 3. The method for forminga conductive pattern on a substrate of claim 2, wherein the resin ismade of a light setting resin, and the resin having self-assembledbetween the convex pattern and the substrate is cured with light byselectively irradiating the resin with light in the third step.
 4. Themethod for forming a conductive pattern on a substrate of claim 3,wherein the flat plate is made of a transparent substrate, and a lightmasking film is formed in a surface area of the flat plate other thanthe convex pattern.
 5. The method for forming a conductive pattern on asubstrate of claim 1, wherein the conductive pattern is made of theaggregate of the conductive particles in which the conductive particlesare in contact with one another.
 6. The method for forming a conductivepattern on a substrate of claim 1, wherein the third step includes asub-step of heating the fluid body at a temperature at which theconductive particles are melted after allowing the fluid body toself-assemble between the convex pattern and the substrate, and theconductive particles are bonded to one another through metallic bond inthe sub-step of heating.
 7. The method for forming a conductive patternon a substrate of claim 6, wherein a melting point of the conductiveparticles is higher than a boiling point of the gas bubble generatingagent.
 8. The method for forming a conductive pattern on a substrate ofclaim 1, wherein the third step includes a sub-step of pressing the flatplate against the substrate after allowing the fluid body toself-assemble between the convex pattern and the substrate, and theconductive particles are contact bonded to one another in the sub-stepof pressing.
 9. The method for forming a conductive pattern on asubstrate of claim 1, wherein the gas bubble generating agent is made ofa material that boils when the fluid body is heated or a material thatproduces a gas through thermal decomposition.
 10. The method for forminga conductive pattern on a substrate of claim 1, wherein the gas bubblegenerating agent is made of two or more kinds of materials havingdifferent boiling points.
 11. The method for forming a conductivepattern on a substrate of claim 1, wherein the third step is performedwhile varying the gap between the substrate and the flat plate.
 12. Themethod for forming a conductive pattern on a substrate of claim 1,wherein at least a surface of the convex pattern is made of a metal. 13.The method for forming a conductive pattern on a substrate of claim 1,wherein the third step includes a sub-step of filling a sealant in thegap between the substrate and the flat plate after allowing the fluidbody to self-assemble between the convex pattern and the substrate andcuring the sealant after filling.
 14. The method for forming aconductive pattern on a substrate of claim 1, wherein the convex patternis formed as a convex pattern having at least two kinds of portions withdifferent heights.
 15. The method for forming a conductive pattern on asubstrate of claim 14, wherein a portion with a smaller width is higherthan a portion with a larger width in the convex pattern.
 16. The methodfor forming a conductive pattern on a substrate of claim 1, wherein thesubstrate is a wiring board, and the conductive pattern corresponds toat least a part of a wiring pattern formed on the wiring board.
 17. Themethod for forming a conductive pattern on a substrate of claim 1,further comprising a step of removing the flat plate after the thirdstep.
 18. The method for forming a conductive pattern on a substrate ofclaim 1, wherein the conductive pattern has a cross-section in a shapeof an hourglass.